What are the best answers for “conducted with or marked by hidden aims or methods”?

clandestine, surreptitious, undercover, underground.

How many letters does the top answer for “conducted with or marked by hidden aims or methods” have?

The most common answer is clandestine (11 letters).

4 answers found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

clandestine 11 letters wn-def

conducted with or marked by hidden aims or methods

surreptitious 13 letters wn-def

conducted with or marked by hidden aims or methods

undercover 10 letters wn-def

conducted with or marked by hidden aims or methods

underground 11 letters wn-def

an electric railway operating below the surface of the ground